Cellular systems using spread spectrum encoding are known. Direct sequence code division multiple access (DS-CDMA) and frequency hopping (FH) are the two most well known of the spread spectrum technologies in which an information signal is distributed over a relatively wide spectral area for purposes of reducing the impact of interference. Frequency hopping achieves the benefits of spread spectrum transmission by hopping through a number of conventional narrowband channels, thereby reducing the average impact of interference in any one channel or group of channels.
DS-CDMA spreads an information signal over a designated spectrum by modulating the information signal with a spreading code which has properties similar to that of a random number. The spreading code used is typically the output of a pseudorandom number generator which provides a number sequence that repeats over a relatively long time interval.
At a receiver the DS-CDMA spread spectrum signal must be de-spread through use of a de-spreading code that has the same characteristics as the spreading code. De-spreading is accomplished by the correlation of the received spread signal with a synchronized replica of the spreading code used to spread the transmitted information.
The use of a spreading code that has the characteristics of a random number ensures that the transmitted signal will be randomly spread throughout a transmission spectrum. The requirement for a duplicate of the spreading code at a receiver (for de-spreading), on the other hand, requires that the spreading (and de-spreading) code be a repeating sequence known to both transmitter and receiver. The use of identical spreading and de-spreading codes at both transmitter and receiver provides the basis for communication between multiple pairs of communicating parties within the same spectrum with minimal mutual interference.
Within a CDMA cellular system, separate control channels are maintained for system access through a list of spreading codes known to both mobile communication units and base sites. Upon desiring access a communication unit transmits a request for system access. The access request may contain such control information as a user identifier and a called number. The base site may respond with control information identifying a spreading code of a traffic channel and a transmit power level.
Upon receipt of the traffic channel information, the communication unit synchronizes to the traffic channel and begins exchanging information with another user at the called number. Control information between the base site and the communication unit during maintenance of the traffic channel is exchanged through the use of "blank and burst" or "dim and burst" techniques.
During "blank and burst" or "dim and burst" an audio signal is muted or encoded at a lower bit rate, respectively, while control information is exchanged between base site and communication unit. Since the exchange of control information occurs very rapidly the disruption in audio signals between users is slight. Audio information that would have been exchanged during "blank and burst" or "dim and burst" is simply discarded. The use of variable rate vocoding and muting techniques means that this "stealing" of bits of speech is undetectable by users.
While "blank and burst" and "dim and burst" are effective during exchanges of audio information, the impact upon data exchanges is far more serious. For transparent services, data information will be lost with the sending terminal equipment being unaware of the loss and the receiving terminal equipment being unable to recover the lost data. Recovery is impossible within transparent services because, by the very nature of transparent services, no provision is made to provide for error protection and re-transmission upon the detection of errors.
For non-transparent services, the use of end-to-end protocols provides a means through which terminal equipment will be able to recover by re-transmission handshaking. The result, on the other hand, is a loss in throughput when the receiving terminal detects an error and request re-transmission of missing frames. Because of the importance of data transmission, a need exists for a means of exchanging control information between base sites and communication units in CDMA systems that does not result in a loss of user information.